Agrobacterium elicits neoplastic growths on many plant species. This genetic modification results from the transfer and integration into the plant genome of a single-stranded copy (T-strand) of the bacterial transferred DNA (T-DNA) from the bacterial tumor-inducing (Ti) plasmid. In planta expression of several oncogenic genes encoded by the T-DNA leads to formation of tumors1 and production and secretion of specific amino acid and sugar phosphate derivatives (opines) which are then exclusively utilized by the bacterium as a carbon/nitrogen source. Plant genetic transformation by Agrobacterium requires the presence of two genetic components located on the bacterial Ti plasmid: (i) T-DNA, the actual genetic element transferred into the plant cell genonme, and (ii) the virulence (vir) region composed of seven major loci (virA, virB, virC, virD, virE, virG, and virH), encoding most components of the protein machinery mediating T-DNA transfer (recently reviewed in refs. 2, 3-5).
The ability of Agrobacterium to infect eukaryotic cells is not limited to plants, and, in laboratory conditions, Agrobacterium has been shown to genetically transform yeast6-8, filamentous fungi and cultivated mushrooms9, and human cells10. Thus, Agrobacterium has been utilized as a model organism capable of a wide-range trans-kingdom DNA transfer. Furthermore, disarmed Agrobacterium strains that lack the wild type T-DNA, are widely used in plant genetic engineering11-13. One of the long-standing goals of these basic scientific and applied aspects of the Agrobacterium research is the increase of the transformation efficiency. To date, this objective has been addressed by modifying the Agrobacterium itself, e.g., introducing multiple copies of various vir genes14-16, or by optimizing tissue culture and inoculation techniques17.
While only the wild-type Agrobacterium T-DNA carries tumor-inducing genes, any DNA placed between the T-DNA borders will be transported into the plant cell nucleus (reviewed in ref. 19). This lack of sequence specificity implies that a T-DNA molecule itself does not contain specific signals for nuclear import and integration. Instead, this process is likely mediated by two Agrobacterium proteins, VirD2 and VirE2, which are thought to directly associate with the T-strand, forming a transport (T) complex20, 21. VIP1 (VirE2-interacting protein 1) is a recently discovered Arabidopsis protein, which is required for Agrobacterium infection.18 Nuclear import of the T-complex is most likely assisted by VIP1 that specifically interacts with VirE218 and may mediate its interaction with the nuclear import machinery of the host cell22.
Prior to the present invention, no endogenous host factors have been described that improve the Agrobacterium-mediated gene transfer. The present invention provides methods for increasing host susceptibility to Agrobacterium infection by overexpressing in the host, the Arabidopsis VIP1 or other VIP1-like gene.